This invention relates to an apparatus for controlling the air/fuel ratio of an air/fuel mixture supplied to an internal combustion engine associated with an evaporated fuel purging unit.
It is the current practice to avoid discharge of fuel evaporated in the fuel tank to the atmosphere with the use of a canister having an absorbent therein for accumulating the fuel vapor introduced from the fuel tank into the canister. Fresh air is introduced into the canister to purge the accumulated fuel vapor from the absorbent and introduced the purge (or purged) fuel vapor, along with the fresh air, into the engine induction passage. In order to correct deviations of the air/fuel ratio from stoichiometry due to variations and changes in the fuel injectors and airflow meter with time, the air/fuel ratio is learned to update the last air/fuel ratio for air/fuel ratio feedback control. During the learning control, however, an error will be introduced into the learned air/fuel ratio value when the fuel vapor is introduced from the canister to the engine.
For example, Japanese Patent Kokai No. 4-109050 discloses an air/fuel ratio control apparatus adapted to inhibit the air/fuel ratio learning control when the fuel vapor is purged from the canister and introduced into the engine. This condition is judged when the rate of temperature decrease of the absorbent placed in the canister exceeds a predetermined value. However, the air/fuel ratio learning control is influenced not only by (1) fuel vapor purged from the canister, but also by (2) fuel vapor introduced from the fuel tank into the engine without absorption in the canister. The second case occurs at high fuel temperatures and cannot be judged from the absorbent temperature.